1. Field of the Invention
The present invention relates to improvements in a photo coupler and an array thereof.
2. Description of the Prior Art
An optical coupling device including a light emitting diode and a semiconductor photo responsive device (hereinafter referred to a photo coupler) has been used for the isolation of solid state relays and transmission lines. Also, a photo coupler array consisting of a plurality of photo couplers mounted on a same substrate or a combined circuit consisting of a photo coupler and an integrated circuit are very advantageous for a logic circuit and an audio circuit.
One type of prior art structure of a photo coupler is constructed of a photo responsive device mounted on an insulating substrate and electrically connected to the conductive layer of a predetermined pattern formed on one surface of the insulating substrate for the photo responsive device.
A light emitting diode is placed above the photo responsive device with a transparent insulator disposed therebetween and is electrically connected to the conductive layer for the light emitting diode formed on one surface of the insulating substrate by a lead wire.
Another prior art structure of the photo coupler is constructed as follows. A photo responsive device and a light emitting diode are respectively mounted on both surfaces of an insulating substrate having a penetrating hole so that the light emitting diode faces the photo sensitive surface of the photo responsive device, and both the diode and the device are electrically connected to the respective conductive layer of predetermined pattern formed on both surface of the insulating substrate.
These two types of prior photo couplers have many defects as follows. The light emitted only from the one surface of the light emitting diode that faces the photo responsive device effectively arrives at the photo sensitive surface of the photo responsive device. Namely, the prior photo coupler did not fully utilize the light emitted from other surfaces of the light emitting diode, so that the light transmission efficiency of the prior photo couplers between the light emitting diode and the photo responsive device was relatively low.
When a plurality of photo couplers are situated close to each other, or a photo coupler and devices are integrated in an integrated circuit erroneous operation thereof is caused by the light emitted from the other surface of light emitting diodes. Consequently, an increase of the degree of integration of the photo coupler array has been limited.
Further, the mechanical strength of the prior photo coupler was weak, because the light emitting diode, the photo responsive device, and the electrical connections therefor were externally exposed. Still further, when the electrical connections between a light emitting diode and a conductive layer are formed with wires such as gold wires of about 50 .mu..phi., the burn-out of the wires during the handling at the production process reduces the reliability of the product.
For increasing the light transmission efficiency of the photo coupler, the photo responsive device must be located at the portion where the intensity of the light emitted from the diode is at a maximum. In prior art types of photo-coupler structures, the light emitted from the light emitting diodes does not necessarily impinge upon the area of the photo-responsive devices where light intensity is a maximum, because accurate positioning of the photo-responsive device just above the light emitting diode is very difficult. Mass productivity of the photo coupler is limited for this reason.
In the photo coupler array wherein the conductors for each photo coupler and other devices are closely formed, the electrostatic capacitance between the conductive layers for the light emitting diode and the photo responsive device should be as small as possible. But in the prior art structures of the photo coupler array, especially in the type where the conductive layers are formed on only one surface of the substrate, the electro-static capacitance mentioned above is necessarily large and the breakdown voltage is insufficient, which results a limited degree of integration of the photo coupler array.